Photoelectric smoke detectors in residential and commercial buildings include a smoke chamber, a light source, a carbon monoxide sensor, and a photodetector. When smoke from an object enters the smoke chamber, it affects the photodetector output, which is used to determine a concentration of smoke in the chamber. The smoke concentration is evaluated together with the carbon monoxide concentration to determine if the smoke is associated with an emergency event or a non-emergency event. If the event is an emergency event, the smoke detector generates a warning alarm. Evaluation of the smoke concentration together with the carbon monoxide concentration does not allow for an emergency event to be distinguished from a non-emergency event in all cases.